Water-in-oil emulsion blasting agents are known and were first disclosed in U.S. Pat. No. 3,447,978 to Bluhm. These explosives demonstrated in a three-inch diameter high velocities of detonation, typically exceeding about 17,000 feet per second. These emulsions were rendered detonable by incorporating occluded gas or voids to make the explosive sensitive to detonation by a booster charge. The density of the explosive was decreased by occluded gas or the inclusion of density-reducing agents such as closed-cell void containing materials. For example, microballoons were used.
Subsequent to the general development of water-in-oil emulsion explosives, water-in-oil emulsion explosive compositions containing solid particulate ammonium nitrate ("AN") or ammonium nitrate fuel oil ("ANFO") were developed for use in large diameter bore holes typically larger than four inches in diameter. Such compositions are illustrated in U.S. Pat. Nos. 4,555,278; 4,111,727; and 4,181,546. The addition of solid oxidizer salts generally reduces the velocity of the water-in-oil emulsion explosive but the velocity remains high enough to be useful and in particular, the lower velocity is beneficial for heaving and mining of softer rock and ore formations such as in strip mining of coal. These blended water-in-oil emulsion compositions containing solid particulate oxidizer salts, AN or ANFO, are typically mixed together at the site at which they are employed and detonated rather quickly, that is, generally within less than 24 hours.
Although dynamite explosives become less sensitive to detonation as the diameter decreases, in the mining industry they were continued to be utilized after the development of water-in-oil emulsions particularly in underground operations and in very hard rock formations. However, the industry has always been interested in the replacement of dynamite by a suitable explosive which is less hazardous to manufacture, cheaper, and yet provides the performance characteristics of dynamite. Dynamite was and still is extensively used in operations where bore hole diameters are less than 2.5 inches. In response to the need to provide a suitable substitute for dynamite, Atlas Powder Company developed Powermax. These compositions are generally disclosed in U.S. Pat. No. 4,110,134 to Wade. Wade developed small diameter, typically 1.25 inches and less, explosives which were reliably detonable by a #6 blasting cap. These compositions have, to some extent, replaced dynamite.
In addition to water-in-oil emulsions so-called melt-in-fuel or anhydrous emulsions can also be utilized. These emulsions are described in U.S. Pat. No. 4,248,644 to Healy. They are similar to water-in-oil emulsions except that they contain no water in the oxidizer or discontinuous phase of the emulsion. Further, melt-in-fuel emulsions do not contain water in the fuel phase.
Explosives are selected for use in particular applications depending upon the result desired. For example, in highway construction when cutting through mountains, it is generally desirable to use a high velocity explosive which creates a shock wave which completely fractures a rock. This will then produce a face which is relatively intact and less likely to cave in. In contrast, in other applications, it is desirable to obtain heaving and fracturing of the rock. This result is generally accomplished by a lower velocity explosive. Thus, in open pit mining it is more preferred to use a heaving explosive which pushes the ore away from the face and also pulverizes the ore. This allows the ore to be more easily removed and processed. Also, in the selection of explosives, the power possessed by volume of an explosive is important to achieve the best efficiency in the mining operations. With water-in-oil and melt-in-fuel emulsion explosive compositions, the velocity and power of explosive compositions can be affected in a positive manner by the addition of certain solid components. However, the drawback to the addition of solids to water-in-oil and melt-in-fuel emulsion explosives is that the solids tend to destabilize the emulsion. A tendency to destabilize indicates that the emulsion will break, i.e., that the discontinuous phase will not remain dispersed throughout the continuous phase. When the emulsion breaks, the explosive becomes less sensitive to detonation and, depending on the degree of breakdown, may become nondetonable. Further, generally upon breaking the emulsion becomes hard and is harder to handle, i.e., pump or auger. Stability is a very important factor in small diameter water-in-oil and melt-in-fuel emulsions intended to be replacements for dynamite, as they are generally prepackaged and thus must have a sufficient shelf-life in which they retain their desired properties.
It was generally believed that the solids caused instability of water-in-oil emulsions by causing migration of the water from the dispersed aqueous phase in the emulsion and destabilizing the droplets such that the emulsion would break. The approach taken previously to make the solids more compatible with the emulsion was to coat the solids with a waterproof coating such as waxes or film forming plastics, for example cellulose acetate butyrate. For example, see U.S. Pat. No. 4,555,271. This approach has obtained only limited success. In contrast, no explanation has been given for the destabilization of melt-in-fuel or anhydrous emulsion explosives. That is, since the anhydrous emulsion contains no water in the dispersed phase the explanation given for the breakdown of water-in-oil emulsions does not apply.
The present invention yields the technical advantage of providing coatings for solid components which increase the stability of water-in-oil and melt-in-fuel emulsion compositions into which the solids are incorporated without the use of waterproofing agents.